1. Field of the Invention
The present invention relates to an information recording medium wherein a reproducing apparatus uses a relative motion with respect to the recording medium to read information, and particularly to an information recording medium wherein optical means are used to carry out recording and/or reproducing process.
2. Description of the Related Art
Currently there existed various systems which read information with relative motion of an information recording medium, and these systems use recording medium in shapes of a disk, a card, or a tape. The information recording medium in the form of a disk, especially with an optically recording and/or reproducing system, is heavily involved in our daily life. For example as a reproduction-only information recording medium which uses light with wavelength of 650 nm, there are DVD videos with pre-recorded visual information, DVD-ROMs with pre-recorded programs and such, and SACDs and DVD audios with pre-recorded audio information. In addition there are DVD-Rs as an addable recording/reproducing information recording medium using dye, DVD-RAMs and DVD-RWs as a recording/reproducing information recording medium using phase change, and ASMO, did, and GIGAMO as a recording/reproducing information recording medium using magneto-optics.
Shortening the wavelength of a laser has been researched over the years in order to improve the recording density. The recording density is determined by MTF (spatial frequency) of an optical system, and it is expressed by the equation λ/4NA (λ: reproduction wavelength, NA: numerical apertures of an objective lens). Since a pit length (or a mark length) at the reproduction limit is λ/4NA, reducing λ and increasing NA is necessary in order to increase the recording density.
Recently invented gallium nitride compound semiconductor light emitting element (Japanese Patent No. 2778405, for example) emits light near λ=405 rm. It seems to be a very important light-emitting element to significantly increase the recording density. The research for corresponding objective lens for the wavelength near the similar range is also progressing, especially with NA over 0.7 under development.
Development of such information recording medium reproducing apparatus with small λ and NA over 0.7 is also progressing, and combination of these technologies gives hope to developing an optical disk with significantly increased recording capacity compared with the current DVDs.
A research to reduce the thickness of a transmission layer to transmit light, as NA becomes larger, to reduce aberration in case the information recording medium tilts is concurrently progressing.